Synthetic aperture radar (SAR) systems in general are well known. Further, motion-compensated synthetic aperture radar systems have been in development and use for many years.
One object of such systems is to generate terrain maps from a moving platform in flight. The map generated thus describes a selected portion of the terrain observed by an aircraft for example.
The radar system can be adjusted by the pilot as desired, within certain bounds of course, to determine the precise location and size of the patch or portion of terrain being observed, mapped or monitored. Generally, the terrain to be mapped is below the aircraft and off to the side. The aircraft typically flies at a given altitude and variable velocity, and its radar has a downward angle of view.
The radar transmits repeated radio frequency (RF) pulses at a selected pulse repetition frequency (PRF) or its inverse, the pulse repetition interval (PRI), toward the terrain to be mapped. After each pulse transmission, the radar receiver waits to receive any return from the selected terrain.
Since the radar has been set to receive a return from a given, preselected general region--the selected map area--and the radar signals travel at a known velocity, it is known precisely when a return is likely to come from terrain at a given range. In fact, the expected time of return of signals from subdivisions within the area to be mapped can also be precisely determined.
Accordingly, for each transmitted pulse, depending upon the terrain, there will be return corresponding to a preselected number of range portions of the area to be mapped. It is of course not possible initially to resolve precisely from which angular location the return comes. Such resolution is however possible after the data for each range bin has been spectrally analyzed (as by Fourier transform for example).
Nonetheless, meaningful information is acquired in the form of a complex number representative of the return phase and magnitude for a predetermined number of range bins in the selected terrain area. Those complex numbers, representing the return over time derived from a single output pulse, are representative of all of the range bins covering the selected terrain.
As the radar platform continues flight, further pulses are transmitted and their return is received. For each subsequent transmitted pulse, a complex return is received, and this in turn is stored in a corresponding range bin. In the end, a complex matrix will have been established, which contains bundles of information regarding the terrain at selected ranges for the map region selected.
As suggested above, in order to map the terrain, these bundles of information need to be analyzed as to angle or location. This is done by Fourier transforms, which convert the information into frequency information, thereby permitting the angle and amplitude of the return to be established. There will be a separate Fourier transform performed for each range bin.
To correct for the changes in position of the radar platform over time--that is to correct for the change in position between successive pulse transmissions, which affects the phase of the return--it is necessary to perform a function on the return signals known as focusing, or motion compensation. This process causes a single point on the map, known as map reference center, to come into sharp focus.
This is done conventionally by placing accelerometers in the moving radar platform. These accelerometers provide signal indications of the changes in motion during spatial translation of the platform. In other words, acceleration information is produced. This does not, of course, directly specify the new location to which the platform has traveled.
To accomplish this, two integrations of the acceleration information are, in effect, constructed. First, the acceleration components in each of the standard coordinate directions are integrated to provide corresponding velocity information. Then another integration is performed upon the velocity information to produce actual location indications with respect to the standard coordinate directions.
From this new position information, a phase correction can be applied to signals coming from the same map location, which correction corresponds to the change in range. By applying this correction, the effective range from the radar center or platform to the map reference center is forced to be equal for successive radar pulses. As a result, the return from an object located at the map reference center will appear in the radar map as a point in sharp focus.
When all of the information regarding the same terrain portion for a sufficient number of successive pulses is analyzed by Fourier transform, the resulting frequency information is translated into phase and field angle information. Thus, the intensity of return for a specific map location can be determined.
Alternately, the motion information, instead of being computed for a preselected map reference center from accelerometer measurement data, can be extracted from the radar returns of prominent radar scatterers. This method is known as "autofocus".
According to the prior art, producing radar pulses at a regular pulse rate intervals is well known. Moreover, setting the carrier frequency of the transmitted pulses to a specific predetermined value is well known.
It is also well known, however, that the geometric extent of the focused area of maps produced by current synthetic aperture radar systems is reduced by changes in aircraft movement and flight maneuvers. In other words, established SAR mapping techniques are subject to severe limitations in terms of depth of focus, when the radar platform moves abruptly.
Two methods have been used to mitigate the reduction in depth of focus under accelerated aircraft motion. According to one method, by using standard focusing techniques, a multiplicity of SAR maps are produced of adjacent areas, each having a depth of focus limited by the aircraft acceleration. The size of the resulting map defines the extent of the increase in the depth of focus.
Such an approach, which uses a multiplicity of map reference centers (or, in the case of "autofocus", of prominent scatterers) is, however, typically expensive in terms of hardware and/or software development costs as well as in processing time. Alternately, a variation of the radar pulse repetition interval (PRI) or its inverse, the pulse repetition frequency (PRF) is used.
The prior art of PRF variation does include use of more than a single focal point; the location of a second point cannot, however, be selected, but it is determined by the nature of the particular algorithm for PRF variation.